OFDM is a type of modulation that is widely used in high data rate wireless transmission because it allows to transform a wideband channel in a set of parallel narrowband channels, which greatly simplifies equalisation, channel estimation, resource allocation etc. For low mobility and large packet size wireless transmission, spectral efficiency can be increased by expanding the OFDM symbol size. In DVB-T2 (digital video broadcasting-second generation terrestrial) and ultra-high-definition TV (UHDTV or super hi-vision, SHV), the OFDM symbol size can be as large as 215 subcarriers. One of the drawbacks of a very large OFDM symbol size is the increased peak-to-average power ratio (PAPR) of the time-domain signal. A large PAPR causes nonlinearity problems and power losses at the power amplifier (PA) as this is forced to operate at lower efficiency.
The performance of a power amplifier (PA) in an OFDM system is greatly affected by the ratio between peak and average power (PAPR) in the time domain signal and the probability distribution of the transmit signal. These parameters are all the more critical in OFDM systems with a large number of subcarriers such as existing and future digital TV broadcasting systems like DVB-T2 and Super Hi-Vision (SHV), where the OFDM symbol size can scale up to 32 K subcarriers.
A number of different techniques have been developed for PAPR reduction in OFDM systems. One strategy for PAPR reduction is tone reservation (TR), whereby a set of subcarriers are not used for data but reserved for a PAPR reduction dummy signal that is orthogonal to the data signal. Through the use of reserved tones the data signal is unaffected. Several techniques are available to calculate the signal for the reserved tones. In one algorithm signal peaks are cancelled iteratively in the time domain by using a set of impulse-like kernels made available by the reserved subcarriers. Another technique, TR using active sets (TR-AS; Krongold, B., Jones, D. (2004, February), An Active-Set Approach for OFDM PAR Reduction via Tone Reservation, IEEE Transactions on Signal Processing, 52(2), 495-509), operates by iteratively cancelling signal peaks by adding a new peak to the set of peaks to be reduced simultaneously at each iteration.
Although some of these techniques achieve good performance, they offer diminishing returns for increased complexity. Therefore, PAPR tends to reach a plateau after the first few iterations and no significant further reduction results from allowing more complexity. Moreover, it is not clear how far this plateau is from optimal, i.e., what is the lowest possible PAPR that can be achieved with a given number of reserved tones and limit on the average power per tone of the TR signal.